Chlorinated solvents are the most common class of groundwater contaminants found at hazardous waste sites in the United States. In a list of the top 25 most frequently detected contaminants at such sites, the Agency for Toxic Substances and Disease Registry (ASTDR) found that ten of the top twenty contaminants were chlorinated solvents or their degradation products. National Research Council, Alternatives for Groundwater Cleanup (National Academy Press, Washington, D.C. 1994). The same survey found that a common contaminant, trichloroethene (TCE) is present at more than 40% of the National Priority List sites. Remediation of groundwater contaminated by these compounds presents unique obstacles related to their inherent characteristics, including hydrophobicity and high density. Recent advances in the understanding of biodegradation processes involving chlorinated solvents permit remediation of residual contamination source areas in low permeability, saturated or variably saturated soils at a much lower cost than conventional methods. One example of such development is provided in U.S. patent application Ser. No. 12/049,959, which is incorporated by reference into this disclosure in its entirety.
Metals, perchlorates, explosives, and other contaminants also appear on lists of contaminants frequently detected at hazardous waste sites. Volatile organic and semi-volatile petroleum hydrocarbons and other associated contaminants are significant contaminants related with the petroleum exploration, production, transportation, processing, and distribution industries.